Measurement of total mirtazapine and normirtazapine in plasma/serum by liquid chromatography with fluorescence detection

A simple high performance liquid chromatography (HPLC) method for the measurement of the new antidepressant mirtazapine and its N-demethyl metabolite, normirtazapine, in human plasma or serum during low dose mirtazapine therapy has been developed. A Waters Spherisorb S5 SCX column was used with ammonium perchlorate (50 mmol/l) in methanol/water (95 + 5 (v/v)), apparent pH 6.7, as eluent, and fluorescence detection. Only small volumes of sample (0.2 ml) and extraction solvent are used. An interference study found no significant co-elution with drug or metabolite, although paroxetine co-elutes with the internal standard. The recovery of mirtazapine and normirtazapine (mean±S.D.) was 79±2, and 64±3%, respectively. The LOD was estimated as 0.5 μg/l, LLOQ was 1 μg/l, with a linear response over the concentration range 4–1000 μg/l (both analytes). The analytes were stable in serum for at least 10 months when stored at −20 °C. Intra- and inter-day accuracy were in the range 91–107 and 93–103%, respectively. In clinical samples (n=14, median mirtazapine dose 45 mg per day, range 15–45 mg per day) the median (range) mirtazapine and normirtazapine concentrations were 26 (8–40) and 21 (8–32) μg/l, respectively.